Mark H Weaver <m...@netris.org>:
> Marko Rauhamaa <ma...@pacujo.net> writes:
>
>> A PF_UNIX socket can be used to pass an open file descriptor between
>> processes. However, the relevant control message seems to be missing
>> in Guile's POSIX facilities (send only accepts a data message).
>
> Indeed, we lack that functionality in core Guile.
>
>> Am I mistaken, and are there any plans to add control message support to
>> Guile's library?
>
> I'm not aware of any plans to add it, but if we can agree on an API
> and someone contributed a suitable implementation, I don't see why we
> couldn't add it.
The C API is really messy. However, after looking at it from different
angles, I'm thinking one shouldn't deviate too far from it. Here is the
germ of a Guile API proposal:
sendmsg NAME BYTEVECTOR-INPUT-PORT [CMSG-LIST [flags]]
=> integer
NAME is a placeholder and should be set to #f.
BYTEVECTOR-INPUT-PORT may be #f. It captures the idea of the gather
array.
CMSG-LIST may be #f. Otherwise, it is a list or lists. The member lists
have three elements: level, type, bytevector.
To support creating the CMSG-LIST elements portably, auxiliary functions
are provided:
cmsghdr:make-socket-rights list-of-ports-and/or-fds
=> bytevector
cmsghdr:make-credentials pid uid gid
=> bytevector
For example:
(sendmsg #f #f (list (list SOL_SOCKET
SCM_RIGHTS
(cmsghdr:make-socket-rights (list input-fd)))))
For the other direction:
recvmsg [BYTEVECTOR-OUTPUT-PORT [flags]]
=> values COUNT NAME CMSG-LIST
BYTEVECTOR-OUTPUT-PORT may be #f. It represents the scatter array.
NAME is a number or #f.
CMSG-LIST may be #f. Otherwise, it is as above.
To support interpreting the CMSG-LIST elements portably, auxiliary
functions are provided:
cmsghdr:socket-rights bytevector
=> list-of-fds
cmsghdr:socket-credentials bytevector
=> values pid uid gid
For example:
(call-with-values
recvmsg
(lambda (count name cmsg-list)
(let loop ((cmsgs cmsg-list) (fds '()))
(if (null? cmsgs)
fds
(loop (cdr cmsgs)
(let ((cmsg (car cmsgs)))
(if (and (= SOL_SOCKET (car cmsg))
(= SCM_RIGHTS (cadr cmsg)))
(append fds (cmsghdr:socket-rights (caddr cmsg)))))
fds)))))
Marko
